A method and a device for temporarily blocking the flow of fluid in a pressurized pipe while retaining a hermetic seal are set forth in U.S. Pat. Nos. 4,949,744 and 5,400,814 which are incorporated herein by reference. In the method and device shown in U.S. Pat. No. 5,400,814, an expandable body, in the form of a rubber balloon is introduced into the pipe and made to expand and block the pipe. The rubber balloon is only kept in place by friction against the inner wall surface of the pipe. This reduces the usefulness of the device when the fluid has a high pressure and when there are pressure surges in the pipe system. They could easily cause the rubber balloon to slide inside the pipe and ultimately burst. Moreover, the inner wall surface of welded pipes is not always smooth and can have welding seams with sharp burrs easily capable of damaging a slipping balloon and causing the balloon to burst.